Metallogeny and tectonomagmatic setting of Ni-Cu magmatic sulfide mineralization, number I Shitoukengde mafic-ultramafic complex, East Kunlun Orogenic Belt, NW China

Accepted Manuscript Metallogeny and tectonomagmatic setting of Ni-Cu magmatic sulfide mineralization, Number I Shitoukengde mafic-ultramafic complex, East Kunlun Orogenic Belt, NW China Zhaowei Zhang, Yalei Wang, Bing Qian, Yuegao Liu, Dayu Zhang, Pengrui Lü, Jun Dong PII: DOI: Reference:

S0169-1368(17)30779-5 https://doi.org/10.1016/j.oregeorev.2018.04.027 OREGEO 2571

To appear in:

Ore Geology Reviews

Received Date: Revised Date: Accepted Date:

16 October 2017 22 April 2018 24 April 2018

Please cite this article as: Z. Zhang, Y. Wang, B. Qian, Y. Liu, D. Zhang, P. Lü, J. Dong, Metallogeny and tectonomagmatic setting of Ni-Cu magmatic sulfide mineralization, Number I Shitoukengde mafic-ultramafic complex, East Kunlun Orogenic Belt, NW China, Ore Geology Reviews (2018), doi: https://doi.org/10.1016/ j.oregeorev.2018.04.027

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Metallogeny and tectonomagmatic setting of Ni-Cu magmatic sulfide mineralization, Number I Shitoukengde mafic-ultramafic complex, East Kunlun Orogenic Belt, NW China a

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Zhaowei Zhang , Yalei Wang , Bing Qian , Yuegao Liu *, Dayu Zhang , Pengrui Lü , Jun Dong a

Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR,

Xi’an Center of Geological Survey, China Geological Survey, Xi’an 710054, Shaanxi, China b

School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China

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No. 108 Geological Team of Sichuan Provincial Bureau of Geological Mineral Exploration and Development, Chongzhou 611200, Sichuan, China

Abstract: The number I Shitoukengde mafic-ultramafic complex is located in the eastern part of the East Kunlun Orogenic Belt (EKOB). It comprises peridotite (dunite and lherzolite), olivine websterite, pyroxenite (websterite and clinopyroxenite), and gabbro. The Ni-Cu sulfide mineralization is hosted in the peridotite and olivine websterite. Zircon SHRIMP U-Pb isotopic analyses reveal an early Carboniferous age of 333.9 ± 4.2 Ma for the olivine websterite and a Silurian age of 424.7 ± 3.7 Ma for the gabbro. Based on these data and the regional tectonic setting, we suggest that the number I Shitoukengde ultramafic rocks formed during the opening of the Paleotethys Ocean in the East Kunlun area. This geological setting is quite different than that of the Xiarihamu Ni-Co deposit (411 Ma) in the EKOB, which formed in a post-collisional environment. The whole-rock Ni contents of the peridotite (dunite and lherzolite) and olivine websterite are negatively correlated with the FeO contents and positively correlated with the MgO and Fo contents of the olivine, indicating that fractional crystallization of the parent magma of the observed rocks did not play a role in the sulfide saturation process. The ultramafic rocks with high Ni contents, i.e., the peridotite and olivine websterite, have low (87Sr/86Sr)i values, indicating low *

Corresponding author:

Email: [email protected] (Yue Gao, Liu), 438 The East of Youyi Road, Xi'an, 710054, China.

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degrees of crustal contamination. The ultramafic rocks with lower Ni concentrations are in contact with the unmineralized marble country rock. We suggest that the assimilation of marble decreased the sulfide solubility of the magmas that produced the number I Shitoukengde complex. The δ34S values of the Shitoukengde sulfide hosted in the ultramafic rocks range from 1.9 to 4.3‰, with an average value of 2.9‰, indicating that crustal sulfur was added to the magma. In the country rocks, we found minor amounts of sulfide only in the gneiss from the Jinshuikou Group. Therefore, the number I Shitoukengde ultramafic rocks were probably contaminated with some sulfur from the gneiss. We propose that the most prospective ultramafic segments of the number I Shitoukengde complex are adjacent to gneissic country rocks.

Key Words: The number I Shitoukengde mafic-ultramafic complex; Early Carboniferous; Zircon SHRIMP U-Pb; East Kunlun Orogenic Belt; Sulfide saturation.

1. Introduction The Ni-Cu sulfide mineralization in the number I Shitoukengde mafic-ultramafic complex (No. I SMC) was discovered by the No. 108 Geological Team of the Sichuan Provincial Bureau of Geological Mineral Exploration and Development in the East Kunlun Mountains (NO. 108GT, 2016). The complex is located in the eastern part of the East Kunlun Orogenic Belt (EKOB) (Fig.

1B). This complex I has some geological characteristics that are similar to those of the recently discovered Xiarihamu giant magmatic Ni-Co deposit in the EKOB. The giant Xiarihamu magmatic sulfide Ni-Co ore deposit contains ~157 million metric tons (Mt) of sulfide ore with average grades of 0.65 wt.% Ni, 0.013 wt.% Co, and 0.14 wt.% Cu (No. 5 GMSQH, 2014). These statistics make the Xiarihamu deposit not only the second-largest Ni deposit in China but also one of the 20 largest known magmatic sulfide deposits in the world (Li S.J. et al., 2012; Li C.S. et al., 2015; Jiang et al., 2015; Song et al., 2016; Zhang et al., 2017). The Xiarihamu Ni-Co deposit is the first sulfide deposit discovered in the EKOB, and the Xiarihamu complex formed in the Late Silurian to Early Devonian (439-406 Ma) (Li C.S. et al., 2015; Jiang et al., 2015; Song et al., 2016). Exploration geologists now wonder whether other mafic-ultramafic rocks in the EKOB have Ni mineralization potential. From 2014 to 2017, the No. 108 Geological Team implemented 2

8 drill holes in the No. I SMC and found some disseminated ore. The ore body hosted in the ultramafic rocks reached its maximum thickness of 120 m in drill hole 4001 (DH4001). The maximum Ni grade of the ore body in the DH4001 is 0.61%, with an average value of 0.32%. Moreover, the U-Pb age of the gabbro in the Shitoukengde complex I is 423.5 ± 3.2 Ma, which is similar to the age of the gabbro in the Xiarihamu complex (Zhou, 2016). Both the Xiarihamu complex and the No. I SMC are located near the Central Kunlun Fault (Fig. 1), and the country rocks of those two complex are similar. Therefore, some exploration geologists think that the Shitoukengde complex I has a great potential for the Ni-Cu mineralization (Zhou, 2016；Zhou et al., 2016; Li et al., 2018). However, at present, Shitoukengde is far from an economic ore deposit. Thus, the Ni-Cu ore mineralization potential of the No. I SMC should be evaluated, and a new exploration strategy should be discussed. Here, we present new geological, geochronological, whole-rock composition, Sr-Nd isotopic and olivine composition data in order to investigate the petrogenesis, metallogeny, and geodynamic setting of this complex.

2. Geologic setting The No. I SMC with Ni-Cu mineralization is located in the EKOB in the northern part of the Qinghai-Tibet Plateau (Fig. 1A). The EKOB is located in the western part of the Central Orogenic Belt in mainland China (Fig. 1A). This belt is bordered by the Qaidam Block to the north, the Bayan Har-Sonpanganzi Terrane to the south, and the Qilian Block to the northeast (Fig. 1B). The E-W-trending EKOB is approximately 1500 km long and 50-200 km wide. This belt is cut by three closely related, deep E-W-trending regional faults (from north to south, the North Kunlun Fault, Central Kunlun Fault, and South Kunlun Fault) (Fig. 1B) (Huang et al., 1984; Jiang et al., 1992). The EKOB is further divided into the Southern and Northern Kunlun zones by the Central Kunlun Fault (Fig. 1B). There are a number of Ni-Cu deposits in the surrounding terrane, including the Jinchuan deposit, which is located in the Alxa Block and is the third-largest magmatic Ni-Cu ore sulfide deposit in the world, and the 441 to 443 Ma Yulonggou and Yuqu Cu-Ni sulfide deposits, which are located in the Qilian Block (Fig. 1B; Zhang et al., 2010; Zhang et al., 2014). The recently discovered Xiarihamu giant magmatic Ni-Co sulfide deposit in the EKOB has been interpreted as the result of magmatism at the margin of the Qaidam Block (Li et

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al., 2015; Song et al., 2016; Zhang et al., 2017). The Shitoukengde mafic-ultramafic complex is located close to the Central Kunlun Fault in the South Kunlun Orogenic Belt, whereas the Xiarihamu deposit is located in the North Kunlun Orogenic Belt, near the Central Kunlun Fault. The Central Kunlun Fault represents the remains of a small Early Paleozoic oceanic basin, and the youngest age of the Qingshuiquan ophiolite complexes in the Central Kunlun is 508 Ma (Li et al., 2006). Furthermore, an arc-related granite in the East Kunlun region has been dated at 508 Ma (CUG, 2003). Therefore, the East Kunlun region entered the oceanic subduction stage after 508 Ma (Mo et al., 2007). The tectonic transition from oceanic subduction to continent-continent collision occurred at 438 Ma (Liu et al., 2013a). The 428 Ma eclogites in the Wenquan area in the eastern part of the EKOB and the 411 Ma eclogite in Xiarihamu-Suhaitu (Fig. 1B) are associated with this continent-continent collisional event (Meng et al., 2013; Qi et al., 2014). In the East Kunlun area, considerable mantle-related magmatic activity occurred after 428 Ma. The products of this activity include Silurian (428-419 Ma) basalt (Zhu et al., 2006; QGS, 2012) and Early Devonian-Middle Devonian (411.5- 382.8 Ma) mafic dykes (Sun et al., 2004; Zhang et al., 2013; Xiong et al., 2014; Yang et al., 2014). This mantle-related magmatic activity was the product of the post-collisional environment (Liu et al., 2012; Liu et al., 2013b; Gan, 2014; Peng et al., 2016; Song et al., 2016). The discovery of the early Carboniferous Buqingshan pillow basalts (340.3 ± 11.6 Ma, Rb-Sr isochron age) and Buqingshan gabbro (332.8 ± 3.1 Ma, LA-ICP-MS zircon U-Pb age) near the South Kunlun Fault (Fig. 1B), as well as the early Carboniferous Mid-Ocean Ridge basalt (Haidewula and Gagangou) in the Central Kunlun Fault (Yuan et al., 1998), indicate the opening of the Paleotethys Ocean in the East Kunlun area (Bian et al., 1999; Liu et al., 2011).

3. Geology of the No. I SMC The Shitoukengde mafic-ultramafic complex is composed of peridotite (dunite and lherzolite), olivine websterite, pyroxenite (websterite and clinopyroxenite), and gabbro (Fig. 2). The country rocks include the gneiss in the Baishahe Formation of the Jinshuikou Group and the metamorphic clastic rocks and marble in the Proterozoic Wanbaogou Group (Fig. 2). The Shitoukengde mafic-ultramafic complex has been divided into three complexs (I, II, and III). The ore bodies exist in complexs I and II. This study focuses on the number I Shitoukengde mafic-ultramafic complex (No. I SMC). The No. I SMC covers an area of approximately 5.7 km2. The ultramafic 4

rocks are irregularly distributed in the gabbro, and the olivine websterite and lherzolite cut through the gabbro (Fig. 3A), confirming that the olivine websterite and lherzolite formed later than the gabbro. Some websterite and olivine websterite veins cut through the lherzolite (Fig. 3C), which suggests that the websterite and olivine websterite formed later than the lherzolite. Therefore, the intrusion sequence of the No. I SMC is as follows: gabbro – peridotite (dunite + lherzolite) – olivine websterite + websterite. The ore body in the No. I SMC is hosted in the peridotite (lherzolite and dunite) (Fig. 2 and Fig. 3). The ore body reaches its maximum thickness of 120 m in DH4001. The ore body is located in the central part of the peridotite lithofacies. The maximum Ni grade of the ore body is 0.61%, with an average value of 0.32%. In the cross section (Fig. 4), ultramafic lithofacies (including peridotites and olivine pyroxenites) are interspersed between the gabbro, which also indicates that the ultramafic lithofacies formed later than the gabbro. The lithofacies in DH4001 include dunite, lherzolite, olivine websterite, websterite, clinopyroxenite and gabbro. The gabbro is located at the bottom of DH4001 (Fig. 5). The gabbro is overlain by the dunite and lherzolite ranging from 235 m to 460 m. The boundary between the dunite and lherzolite is not obvious, indicating that these units formed during the same stage of magmatic evolution. The websterite and clinopyroxenite are distributed from 63.07 m to 235 m above the peridotite (dunite and lherzolite) (Fig. 5). The marble wrapped by the intrusion occurs from 80 m to 95 m. The olivine websterite is distributed in the upper part of DH4001 (Fig. 5). The gabbro is composed of 30-50 vol.% clinopyroxene, 50-60 vol.% plagioclase, and minor hornblende (Fig. 5H). The lherzolite is composed of 65 vol.% olivine, 10-20 vol.% orthopyroxene, 10-20 vol.% clinopyroxene, and minor Cr-spinel (Fig. 5G). The dunite in the central part contains 95 vol.% olivine, 2-4 vol.% orthopyroxene, and 1-2 vol.% sulfide (Fig. 5F). In DH4001, sulfide is only found in the central part of the dunite. The upper part of the dunite is different than the dunite in the central part. The upper dunite does not contain sulfide, but instead contains minor plagioclase (Fig. 5E). Plagioclase mainly occurs interstitially between olivine grains (Fig. 5E). Clinopyroxenite contains 95 vol.% clinopyroxene, 3 vol.% olivine, and minor plagioclase (1-2 vol.%) (Fig. 5D). The websterite comprises approximately 60 vol.% orthopyroxene, 30 vol.% clinopyroxene, and 10 vol.% olivine (Fig. 5C). The olivine websterite contains 35-40 vol.% 5

olivine, 35 vol.% orthopyroxene, and 35 vol.% clinopyroxene (Figs. 5A and 5B). Although disseminated sulfide is only found in the central part of the peridotite (dunite and lherzolite) in DH4001 (Fig. 5F and Fig. 6A), injected massive ore (Fig. 6B) is found in other drill holes. The sulfides (2–4%) mainly include pentlandite (Pn), pyrrhotite (Po), and chalcopyrite (Ccp) (Fig. 6C). Although minor chalcopyrite was found in the Xiarihamu giant Ni-Co deposit, chalcopyrite is common in the No. I SMC. The sulfides in the No. I SMC occur in the interstitial space of cumulus olivine (Fig. 5F) or in the olivine (Figs. 5F and 6C).

4. Sampling and Analytical Methods The zircon crystals used for U-Pb isotopic dating were extracted from an olivine websterite sample (ST-1) from DH4001 (38 m) and a gabbro sample (ST-22) from DH7201 (132 m). The locations of these two samples are shown in Figure 4. Cathodoluminescence and back-scatted electron images were used to select grains with simple zoning patterns for dating. Zircon U-Pb isotopic analysis was carried out on the SHRIMP II ion microprobe at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences. The analytical procedures were similar to those described by Williams (1998). The standards M257 (U=840×10-6, Nasdala et al., 2008) and TEMORA (206Pb/238U age=417 Ma; Black et al., 2003) were used for the calibration of U abundances and 206Pb/238U ratios, respectively. Data processing and assessment were carried out using the SQUID and ISOPLOT programs (Ludwig, 2003). The measured 204Pb values were used to assess the ages of all samples. A total of 17 samples from DH4001, including three olivine websterite, three pyroxenite, and eleven peridotite samples, were selected to constrain the compositional variations in the olivine of the ultramafic lithofacies. The exact location of each sample is listed in Table 2. Olivine compositions were determined using an electron microprobe at Chang'an University. The analytical conditions included an energy of 15 kV, a beam current of 20 nA, a beam diameter of 1–5 µm and a peak-counting time of 20 s. The analytical error was ±2%. The standard materials used in the analysis of Na 2O, K2O, FeO, MgO, P2O5, MnO, Al2O3, CaO, Cr2O3, SiO2, TiO2, NiO, and ZnO were albite, K-feldspar, hematite, forsterite, xenotime, pyrophanite, corundum, wollastonite, Cr-spinel, kyanite, pyrophanite, metallic nickel, and metal zinc, respectively. All materials were in accordance with the National Standard of the People's Republic of China (1998). 6

The standard title is: General specification of X-ray EDS Quantitative analysis for EPMA and SEM. The standard number is: GB/T17359-1998. The detection limits for all elements under such conditions are 0.01%. The analytical error was ±2%.

A total of 6 samples from DH4001, including two olivine websterite, one pyroxenite, and three peridotite samples, as well as one gabbro sample from DH7201, were selected for whole-rock Sr-Nd isotopic analysis. The exact location of each sample is listed in Table 3. High-precision whole-rock Sr and Nd isotopic measurements were performed using a Nu Plasma high-mass-resolution multicollector inductively coupled plasma mass spectrometer (HR MC-ICP-MS) in the State Key Laboratory of Continental Dynamics, Northwest University, China. The procedural blanks were 40 pg for Rb, 300 pg for Sr, 20 pg for Sm, and 60 pg for Nd. The Sr and Nd isotopic ratios were corrected for mass fractionation relative to values of 86Sr/88Sr=0.1194 and

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Nd/144Nd=0.7219, respectively. The measured values for the NBS-987 Sr standard and the

JNdi-1 Nd standard were 87Sr/86Sr=0.710245 ± 16 (2σ, n=8) and 143Nd/144Nd=0.512117 ± 10 (2σ, n=10), respectively. The Rb-Sr and Sm-Nd isotopic compositions of the USGS reference material BCR-2 were measured to monitor the accuracy of the analytical procedures, and the analyses yielded the following values: 46.62 ppm Rb, 367.49 ppm Sr,

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Sr/86Sr=0.704969 ± 3 (2σ), 6.473

ppm Sm, 27.62 ppm Nd, and 143Nd/144Nd=0.512674 ± 10 (2σ).

5. Analytical Results 5.1. Zircon SHRIMP U-Pb ages The cathodoluminescence images of the selected zircon crystals from the Shitoukengde ultramafic and gabbro samples (ST-1 and ST-22) are shown in Figure 7. All of the zircons exhibited a tabular, uniform internal structure, with no obvious girdle characteristics; they thus represent typical zircons of mafic-ultramafic intrusions. The zircon grains from ST-1 have U and Th concentrations of 339–1593 ppm and 237–1204 ppm, respectively, and their Th/U ratios range from ~0.65–1.40 (Table 1). The zircons from the gabbro sample SK-22 have a wider range of Th (113~925 ppm) and U (238~1314 ppm) contents, with Th/U ratios ranging from 0.36 to 0.70. The weighted mean zircon U-Pb age of the Shitoukengde ultramafic rocks is 333.9±4.2 Ma (Fig. 8A), whereas the gabbro yielded an age of 424.7±3.7 Ma (Fig. 8B). The Shitoukengde ultramafic body

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is 90 Ma younger than the surrounding gabbro, indicating that these two lithofacies formed in two independent magmatic events. 5.2. Whole-rock Ni content and olivine composition In this paper, we obtained whole-rock Ni content data from DH4001 from our cooperative institute, the No. 108 Geological Team of the Sichuan Provincial Bureau of Geological Mineral Exploration and Development. The sampling density is one sample per meter, and the results are listed in Supplementary Material 1 and shown in Fig. 9. The olivine compositions of the rocks in DH4001 are provided in Table 2. The stratigraphic variations in the Fo, MgO, FeO, SiO2, NiO and CaO contents of the olivine, together with the variations in the whole-rock Ni contents in DH4001, are illustrated in Fig. 9. The location of the drill hole is shown in Figs. 2 and 4. In DH4001, olivine websterite occurs from 0–63.07 m, and pyroxenite (websterite and clinopyroxenite) occurs from 63.07–235 m. The whole-rock Ni content of the olivine websterite gradually decreases from 0.167 wt.% at the top to 0.013 wt.% at 63.07 m. The Fo, MgO, and NiO contents of the olivine exhibit positive correlations in the olivine websterite (Figs. 9 and 10). These values progressively decrease downward in the stratigraphic section (Fig. 9). The whole-rock Ni and olivine NiO contents have a positive correlation in the olivine websterite (Fig. 9). In contrast, the whole-rock Ni content is negatively correlated with the olivine FeO content (Fig. 9). The whole-rock Ni content of the pyroxenite (websterite and clinopyroxenite) is approximately 0.01 wt.%; this content remains almost unchanged throughout the pyroxenite. The olivines closest to the marble have higher CaO contents (Fig. 9). The Fo and NiO contents in the pyroxenite have a negative relationship (Fig 10). The whole-rock Ni content of the peridotite (dunite and lherzolite) in DH4001 decreases from 0.61 wt.% at 357 m in the central part to 0.035 wt.% at 236 m in the upper marginal zone and to 0.151 wt.% at 461 m in the lower marginal zone. Only 5 samples in the central part of the peridotite have Ni contents that are higher than 0.4 wt.% in the entire drill hole. In Fig. 9, the whole-rock Ni content of the peridotite (dunite and lherzolite) is negatively correlated with the FeO, SiO2 and CaO contents and positively correlated with the MgO and Fo contents of the

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olivine. The peridotite in the central part has the highest Fo and MgO values, indicating that this represents the first place where fractional crystallization occurred. The Fo and NiO contents of the olivine in the peridotite do not have a negative or positive relationship (Fig 10).

5.3. Sr-Nd isotopes The Sm-Nd isotopic compositions of the Shitoukengde rocks are listed in Table 3. The εNd(t) values and initial (87Sr/86Sr)i values of the Shitoukengde ultramafic rocks were calculated using the zircon U-Pb age of 333.9 Ma. The stratigraphic variations in the εNd(t) values and (87Sr/86Sr)i values in DH4001 are illustrated in Fig. 11. The (87Sr/86Sr)i values of the olivine websterite at 25 m and 38 m in DH4001 are 0.708122 and 0.710289, respectively. The olivine websterite samples with high (87Sr/86Sr)i values have low εNd(t) values and low whole-rock Ni contents (Fig. 11). The (87Sr/86Sr)i value of the pyroxenite at 200 m is 0.709986. The ( 87Sr/86Sr)i values of the peridotite in the upper part and the lower part are 0.711302 and 0.708367, respectively. The (87Sr/86Sr)i value of the peridotite at 349 m in DH4001 (representing the central facies of peridotite) is 0.705816, which is much lower than the corresponding values of the peridotite in the upper and lower parts. In Fig. 11, the ultramafic rocks with relatively high Ni contents, the peridotite and olivine websterite, have relatively low (87Sr/86Sr)i values. The εNd(t) values of the different parts of the peridotite are almost the same. The pyroxenite has the lowest εNd(t) value of all of the lithofacies. The εNd(t) and initial (87Sr/86Sr)i values of one gabbro sample (ST-22) studied here that was collected from DH7201 (138 m) and two gabbro samples compiled from Zhou (2016) were calculated using the zircon U-Pb ages of 424.7 Ma and 423 Ma, respectively (Table 3). The εNd(t) values of the Shitoukengde gabbro range from 1.4 to 4.0, while the εNd(t) values of the ultramafic rocks range from -1.9 and -2.7.

6. Discussion 6.1. The discovery of the early Carboniferous Ni-Cu mineralization in the EKOB Previous studies have indicated that the EKOB experienced two periods of mantle-derived magmatic activity represented by Ni-Cu mineralized mafic-ultramafic complexs: one occurred 9

during the Late Silurian-Early Devonian (Zhu et al., 2006; Liu et al., 2012; Liu et al., 2013a), and the other occurred during the Middle-Late Triassic (Luo et al., 2002; Feng et al., 2012; Chen et al., 2013; Li et al., 2014; Liu et al., 2014; Luo et al., 2014; Wang et al., 2014). The results of gabbro U-Pb dating initially indicated that the Shitoukengde Ni-Cu deposit formed at 423.5±3.2 Ma (Zhou, 2016). However, cross section analysis revealed that the ultramafic lithofacies was intruded into the gabbro (Fig. 4) and the ultramafic lithofacies (olivine websterite and lherzolite) cut through the gabbro (Fig. 3A), thus indicating that the ultramafic lithofacies formed later than the gabbro. The zircon SHRIMP U–Pb ages of the olivine websterite hosting the disseminated Ni-Cu ore and the barren gabbro from the Shitoukengde area are 333.9 ± 4.2 Ma and 424.7 ± 3.7 Ma, respectively (Fig. 4 and Fig. 8). Therefore, the ultramafic lithofacies formed 90 Ma later than the gabbro, and the Shitoukengde Ni-Cu deposit formed in the early Carboniferous. If the ultramafic lithofacies and the gabbro represent contemporaneous products, then the gabbro at the margin of the complex would have been contaminated by more crustal material, thus resulting in the gabbro having lower εNd(t) values than the central ultramafic rocks (Liu et al., 2017; Zhang et al., 2017). However, the gabbro at the margin of No. I SMC has much higher εNd(t) values (1.4 to 4.0) than the central ultramafic lithofacies (-2.7 to -1.9) (Table. 3). Therefore, the εNd(t) values may represent evidence that these two lithofacies formed at different times and in different geological settings. The age data reveal that the early Carboniferous is also one of the geological periods in which the Ni-Cu mineralization formed in the EKOB. The Shitoukengde ultramafic rocks is the first discovered early Carboniferous ultramafic rocks with Ni-Cu mineralization in the EKOB. 6.2. Geodynamic significance The Central Kunlun Fault represents the remains of a small Early Paleozoic oceanic basin; the youngest age of the Qingshuiquan ophiolite complexes in the Central Kunlun region is 508 Ma (Li et al., 2006). Furthermore, an arc-related granite in the East Kunlun region has been dated at 508 Ma (CUG, 2003). Therefore, the East Kunlun region entered an oceanic subduction stage after 508 Ma (Mo et al., 2007). The tectonic transition from oceanic subduction to continent-continent collision occurred at 438 Ma (Liu et al., 2013a). The eclogites in the Wenquan area of the Central Kunlun Fault, which have been dated to 428 Ma, are associated with this continent-continent

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collisional event (Meng et al., 2013). In the East Kunlun area, considerable mantle-related magmatic activity occurred after 428 Ma. The products of this activity include Middle-Late Silurian (428-419 Ma) basalt (Zhu et al., 2006; QGS, 2012), Early-Middle Devonian (411.5- 382.8 Ma) mafic dykes (Sun et al., 2004; Zhang et al., 2013; Xiong et al., 2014; Yang et al., 2014), and Late Silurian-Middle Devonian (419.0-391.1 Ma) A2-type granite (Chen et al., 2013; Liu et al., 2013b; Wang et al., 2013; Gan, 2014; Yan et al., 2016). This mantle-related magmatic activity occurred in a post-collisional environment (Liu et al., 2012; Liu et al., 2013b; Gan, 2014; Peng et al., 2016; Song et al., 2016). The end of the post-collisional stage was marked by the development of an intra-continental molasse basin (Turner et al., 1992). Therefore, the molasse formation and continental volcanic eruptions developed in the Late Devonian Maoniushan Formation represent the end of the post-collisional stage during the tectonic development of the Prototethys Ocean. The presence of the early Carboniferous Buqingshan pillow basalts (340.3 ± 11.6 Ma, Rb-Sr isochron age) and the Buqingshan gabbro (332.8 ± 3.1 Ma, LA-ICP-MS zircon U-Pb age) near the South Kunlun Fault reflects the opening of the Paleotethys Ocean in the East Kunlun area (Bian et al., 1999; Liu et al., 2011). The early Carboniferous volcanic rocks in the Central Kunlun Fault contain some mid-ocean ridge basalt (Yuan et al., 1998). Moreover, the ultramafic rocks in the Shitoukengde complex I near the Central Kunlun Fault formed at 333.9 ± 4.2 Ma. Therefore, the formation of the Shitoukengde ultramafic rocks is likely reflects the opening of the Paleotethys Ocean in the East Kunlun area. This geological setting is quite different than that of the Xiarihamu Ni-Co deposit, which formed in a post-collisional environment during the tectonic development of the Prototethys Ocean (Peng et al., 2016; Song et al., 2016). 6.3. The effects of fractional crystallization and crustal contamination on sulfide saturation Magmatic sulfide deposits form via sulfide segregation from mantle-derived mafic-ultramafic magmas (Naldrett, 2004). The distribution coefficients between sulfides and silicates (DSul/Sil) range from 4×105 to 2-3×106 for platinum group elements (PGE) (Mungall and Brenan, 2014), 800 to 4600 for Cu, and 776 to 2300 for Ni (Patten et al., 2013; Li and Audétat, 2015). Therefore, Cu, Ni, and PGEs are highly compatible with sulfides. The occurrence of sulfides in silicate melts

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requires sulfide saturation in the magmatic system. Generally, the sulfur content at sulfide saturation (SCSS) decreases with increasing pressure and decreasing temperature and increases with decreasing SiO2 content and increasing FeO, TiO2, CaO, and H2O contents (Wendlandt, 1982; Mavrogenes and O’Neill, 1999; Liu et al., 2007; Li and Ripley, 2009). Generally, sulfide saturation can be affected by both fractional crystallization and crustal contamination. Previous research suggests that the consumption of FeO during the fractional crystallization of olivine and chromite is an important factor in sulfide saturation (Tao et al. 2007; Wykes et al. 2015; Liu et al., 2017). That is, if no crustal contamination occurs and the magma has enough sulfur, sulfides will form during the fractional crystallization of olivine and chromite. In DH4001, sulfide only occurred in the central part of the peridotite, where the earliest fractional crystallization occurred and the whole-rock Ni contents of the peridotite (dunite and lherzolite) and olivine websterite are negatively correlated with the FeO content and positively correlated with the MgO and Fo contents of the olivine (Fig. 9). Thus, fractional crystallization did not play a role in the sulfide saturation process. Otherwise, the Ni content would have increased, and sulfide would have formed during a later period of fractional crystallization. If fractional crystallization played a role, the olivine NiO content would exhibit a negative relationship with the Fo content (Zhang et al., 2011). However, in both the peridotite and the olivine websterite, Fo does not exhibit a negative relationship with the olivine NiO content (Fig. 10). Therefore, fractional crystallization did not play a role in the sulfide saturation process. The rocks with higher Ni contents in the peridotite and olivine websterite have lower (87Sr/86Sr)i values (Fig. 11), indicating that the rocks with higher Ni contents experienced lower degrees of contamination. Thus, crustal contamination likely played a role in depressing sulfide saturation. The country rocks of the Shitoukengde complex I contain marble, which was also found in DH4001. Therefore, the Shitoukengde mafic-ultramafic rocks probably assimilated some unmineralized marble country rock. The assimilation of marble led to an increase in the CaO content of the magma. The SCSS increases with increasing CaO content (Li and Ripley, 2009). Thus, the assimilation of marble without sulfides decreases the sulfide solubility of a magma (Holwell et al., 2007; Lehmann et al., 2007; Liu et al., 2017). This finding is consistent with the fact that the whole-rock Ni content of the peridotite (dunite and lherzolite) is negatively correlated 12

with the CaO content of the olivine (Fig. 9). Generally speaking, the rocks in the margin experienced a larger degree of assimilation. This probably explains why sulfide only occurred in the central part of the peridotite. Although sulfide is rare in DH4001, the No. I SMC contains injected massive sulfides (Fig. 6B). This may be due to sulfide liquation and differentiation in a deep magma chamber. Sulfide saturation can be achieved by the addition of crustal sulfur (Lightfoot and Hawkesworth, 1997; Sharman et al., 2013; Liu et al., 2015). The δ34S values of the Shitoukengde sulfide range from 1.9 to 4.3‰, with an average value of 2.9‰ (Zhou, 2016). These values are significantly higher than typical mantle values (-1.80‰ to 0.49‰; see Gao and Thiemens, 1993; Labidi et al., 2013), indicating that crustal sulfur was added to the magma. However, we did not find sulfide in the marble; instead, in the country rocks, we found minor sulfide only in the gneiss from the Jinshuikou Group. Therefore, the No. I SMC was probably contaminated by some sulfur from the gneiss. The SiO2 content of the gneiss can reach up to 70 wt.%, and the addition of SiO2 is beneficial to sulfide saturation (Li and Ripley, 2009; Ripley and Li, 2013). The lithofacies in direct contact with the marble probably does not have the potential to form economic Ni-Cu ore. We thus suggest that future explorations should focus on the ultramafic segments adjacent to the gneiss rather than the segments contaminated by marble without sulfides.

7. Conclusions (1) The Shitoukengde ultramafic rocks formed at 333.9±4.2 Ma (early Carboniferous), while

the Shitoukengde gabbro formed at 424.7±3.7 Ma. (2) The formation of the Shitoukengde ultramafic rocks most likely reflects the opening of the

Paleotethys Ocean in the East Kunlun area. (3) The assimilation of marble decreased the sulfide solubility of the magmas in the number I Shitoukengde ultramafic rocks. (4) The most prospective ultramafic segments of the Shitoukengde are adjacent to gneissic country rocks rather than the segments contaminated by marble without sulfides.

Acknowledgments

13

We are very grateful to the Associate Editor Peter C. Lightfoot, Dr. Xueming Yang and an anonymous reviewer for their great help in improving this manuscript. During fieldworks, the senior engineers Li Shi-jin and Tian Cheng-sheng from the Qinghai Geological Survey and Yin Jian-hua from the No. 108 Geological Team of the Sichuan Provincial Bureau of Geological Mineral Exploration and Development, provided much help. This study was financially supported by the Special Fund for Land and Resources Scientific Research of Public Interest (201511020), the National Project of Geological and Mineral Survey (DD20160013), the Natural Science Foundation of Shaanxi Province (2017JM4002), the State Scholarship Fund ([2016]3035-201608610016) from the China Scholarship Council, the China Postdoctoral Science Foundation (2015M582762XB), and the Shaanxi Province Postdoctoral Science Foundation (2017BSHQYXMZZ06).

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Figure Captions

Fig. 1. Regional Geology of the East Kunlun Orogenic Belt (after Xiao et al. 2014). Age resources: (Yuan et al., 1998; Bian et al., 1999; Zhang et al., 2010; Meng et al., 2013; Qi et al., 2014; Zhang et al., 2014; Li et al., 2015; Qian et al., 2015)

Fig. 2. Geologic map of the Shitoukengde mafic-ultramafic complex (modified after NO. 108GT, 2016; Age resource: Zhou, 2016)

Fig. 3. Contact relationships between the major lithofacies (A: websterite vein cuts through gabbro; B: Peridotite vein cuts through gabbro; C: websterite veins invade peridotite).

Fig. 4. Lithofacies distribution and ore body in a cross section of the No. I SMC

Fig. 5. Distribution of lithofacies intersected by drill hole DH4001 and photomicrographs of the main rock types. Ol = olivine, Sul = sulfides, Cpx = clinopyroxene, Opx = orthopyroxene, and Pl = plagioclase. A. Olivine websterite (taken at 38 m in DH4001). B. Photomicrograph of olivine websterite (38 m). C. Photomicrograph of websterite (103 m). D. Photomicrograph of clinopyroxenite (298 m). E. Photomicrograph of dunite containing plagioclase (298 m). F: Photomicrograph of dunite, sulfide in olivine and the boundaries between olivine grains (386 m), the central part of the peridotite (lherzolite and dunite). G: Photomicrograph of lherzolite (450 m).

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F. Photomicrograph of gabbro (615 m).

Fig. 6. Main ore types of the Shitoukengde Ni-Cu ore bodies. Po = pyrrhotite, Pn = pentlandite, Ccp = chalcopyrite. Fig. 7. Cathodoluminescence images of selected zircons from the No. I SMC.

Fig. 8. Zircon SHRIMP U-Pb isotope concordia plot for the No. I SMC.

Fig. 9. The relationship between the Ni content and major element contents in olivine in DH4001.

Fig. 10. The relationship between the NiO contents and Fo in olivine.

Fig. 11. Relationships between depth and the whole-rock Ni content, the whole-rock (87Sr/86Sr)i value and εNd(t) value

Table Captions Table 1. SHRIMP U-Pb isotopic data from the No. I SMC in East Kunlun

Table 2. Compositions of the olivine in the ultramafic rocks from drill hole 4001 (units: %)

Table 3. The Sr-Nd isotopic data from the No. I SMC

Supplementary Captions Supplementary Material 1. Whole-rock Ni contents in drill hole 4001

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Fig. 1. Regional Geology of the East Kunlun Orogenic Belt (after Xiao et al. 2014). Age resources: (Yuan et al., 1998; Bian et al., 1999; Zhang et al., 2010; Meng et al., 2013; Qi et al., 2014; Zhang et al., 2014; Li et al., 2015; Qian et al., 2015)

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Fig. 2. Geologic map of the Shitoukengde mafic-ultramafic complex (modified after NO. 108GT, 2016; Age resource: Zhou, 2016)

Fig. 3. Contact relationships between the major lithofacies (A: websterite vein cuts through gabbro; B: Peridotites vein cuts through gabbro; C: websterite veins invade peridotite).

24

Fig. 4. Lithofacies distribution and ore body in a cross section of the No. I SMC

25

Fig. 5. Distribution of lithofacies intersected by drill hole DH4001 and photomicrographs of the main rock types. Ol = olivine, Sul = sulfides, Cpx = clinopyroxene, Opx = orthopyroxene, and Pl = plagioclase. A. Olivine websterite (taken at 38 m in DH4001). B. Photomicrograph of olivine websterite (38 m). C. Photomicrograph of websterite (103 m). D. Photomicrograph of clinopyroxenite (298 m). E. Photomicrograph of dunite containing plagioclase (298 m). F: Photomicrograph of dunite, sulfide in olivine and the boundaries between olivine grains (386 m), the central part of the peridotite (lherzolite and dunite). G: Photomicrograph of lherzolite (450 m). F. Photomicrograph of gabbro (615 m).

26

Fig. 6. Main ore types of the Shitoukengde Ni-Cu ore bodies. Po = pyrrhotite, Pn = pentlandite, Ccp = chalcopyrite.

27

Fig. 7. Cathodoluminescence images of selected zircons from the No. I SMC.

Fig. 8. Zircon SHRIMP U-Pb isotope concordia plot for the No. I SMC

28

Fig. 9. The relationship between the Ni content and major element contents in olivine in DH4001.

29

Fig. 10. The relationship between the NiO contents and Fo in olivine.

Fig. 11. The relationships between depth and the whole-rock Ni content, the whole-rock (87Sr/86Sr)i value and εNd(t) value

30

Table 1. SHRIMP U-Pb isotopic data from the number I Shitoukengde mafic-ultramafic comple U 点号 Olivine websterite ST1-1 ST1-2 ST1-3 ST1-4

206

Th 206

ppm

Pb* (ppm)

Pb/238U

Th/U

(Ma)

1σ

207

Pb*/206Pb*

±%

207

Pb*/23

462 1125 1314 457 626 238 298 298 455

306 582 925 266 297 113 168 175 164

21.5 52.4 60.3 20.9 28.7 10.6 13.7 13.9 20.2

0.66 0.52 0.7 0.58 0.47 0.48 0.56 0.59 0.36

339.3 340.3 334.5 332.4 334.8 321.6 336 337.9 323.4

5.3 3.6 3.6 4.1 3.9 7.7 4.8 4.5 5.5

0.052 0.055 0.053 0.051 0.052 0.051 0.054 0.052 0.052

4.3 1.6 2.5 4.3 3 4.5 3.5 4.1 3.3

0.388 0.4144 0.389 0.372 0.38 0.362 0.4 0.383 0.37

305

190

13.7

0.62

324

4.9

0.051

7

0.359

Gabbro ST22-1 ST22-2 ST22-3 ST22-4

825 463 582 380

717 350 714 531

48.5 27.5 34.1 22.1

0.87 0.76 1.23 1.4

425.9 430.6 425.4 422.8

4.5 4.7 4.6 4.7

0.0551 0.05672 0.05657 0.05565

1.4 1.4 1.2 1.5

0.5189 0.5403 0.532 0.5201

ST22-5 ST22-6 ST22-7 ST22-8 ST22-9 ST22-10 ST22-11 ST22-12 ST22-13

446 339 1010 825 740 1593 571 466 653

288 237 728 679 500 1204 641 391 556

25.9 20.2 60.3 48.2 44.1 91.6 32.6 27.3 37.7

0.65 0.7 0.72 0.82 0.68 0.76 1.12 0.84 0.85

420.8 433.8 432.8 424.4 432.5 417.6 415.4 423.4 418.4

4.6 4.9 4.6 4.6 4.7 4.3 4.5 4.7 4.5

0.05474 0.0568 0.05555 0.05601 0.05607 0.05422 0.05547 0.0543 0.05489

1.5 2.1 1.1 1 1.3 0.78 1.5 2.1 1.2

0.509 0.545 0.5319 0.5255 0.5365 0.5003 0.5091 0.509 0.5075

ST1-5 ST1-6 ST1-7 ST1-8 ST1-9 ST1-10

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Table 2. Compositions of the olivine in the ultramafic rocks from drill hole 4001 (units % Drillhole 4001 4001 4001 4001 4001

Depth (m) 25 25 25 38 38

SiO2 40.05 41.35 39.95 38.67 40.33

TiO2 0.02 0.01

4001 4001 4001 4001 4001 4001

38 62 62 62 103 128

40.36 40.17 40.17 39.71 39.90 41.46

0.11 0.03

4001 4001 4001 4001

128 128 224 224

39.37 39.93 39.92 38.71

4001 4001 4001 4001 4001 4001 4001 4001 4001 4001

224 284 284 284 290 290 290 298 298 298

38.72 40.00 40.84 40.33 40.21 41.12 40.42 41.55 40.74 41.49

4001 4001

349 349

40.48 40.69

4001 4001 4001 4001 4001 4001

349 362 362 362 362 362

40.03 40.86 39.74 41.22 40.19 39.46

4001 4001 4001 4001 4001 4001 4001 4001 4001

386 386 386 386 386 386 386 397 397

38.90 39.86 39.28 40.33 39.24 39.17 38.99 39.59 39.63

Al2O3

FeO 12.30 13.54 12.84 13.01 12.75

MnO 0.08 0.18 0.37 0.13 0.27

MgO 45.27 46.12 46.15 46.01 45.90

CaO 0.06 0.03 0.06 0.03 0.04

0.02 0.016 0.035 0.012 0.006 0.011

13.47 17.35 16.29 17.06 12.45 16.80

0.32 0.23 0.21 0.28 0.21 0.13

45.32 43.30 42.99 43.87 47.12 43.86

0.04 0.05 0.02 0.03 0.05 0.02

0.012

18.63 17.94 17.49 17.70

0.22 0.08 0.23 0.15

42.65 42.95 43.51 41.63

0.05 0.03 0.05 0.06

0.012

17.17 10.98 11.24 11.32 10.80 11.33 10.48 10.65 11.27 10.83

0.46 0.25 0.00 0.09 0.09 0.07 0.16 0.32 0.20 0.19

41.79 47.69 47.93 47.98 47.41 48.49 46.87 48.37 47.23 48.43

0.04 0.02 0.05 0.03 0.04 0.04 0.05 0.05 0.02 0.05

0.027

11.23 11.34

0.19 0.19

48.35 48.34

0.03 0.03

11.31 11.17 11.16 11.58 10.74 11.23

0.20 0.20 0.27 0.04 0.11 0.01

46.42 48.41 48.34 48.65 48.62 48.18

0.05 0.05 0.03 0.03 0.06 0.04

11.13 10.16 10.75 11.54 10.49 11.00 11.28 11.69 10.48

0.10 0.04 0.17 0.24 0.16 0.09 0.32 0.20 0.14

47.69 47.90 47.52 48.24 47.50 47.83 47.84 48.06 47.22

0.02 0.02 0.05 0.03 0.03 0.05 0.04 0.03 0.04

0.029

0.01

0.01 0.02

0.02 0.01 0.02 0.03 0.01 0.01 0.04 0.02 0.06 0.01

0.021 0.024 0.013 0.017 0.027 0.04

0.01 0.035 0.01 0.04 0.01

0.039

0.02 0.05 0.01

0.021 0.036 0.029 0.026 0.024

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Na2O

K2O

0.02

0.02

0.01

ZnO 0.23 0.23 0.35 0.18 0.17

Ni

0.15

0.1

0.25 0.01

0.2

0.08 0.13 0.16

0.0

0.14 0.27 0.34 0.34 0.24 0.34 0.26 0.39 0.30 0.27

0.1

0.30 0.25

0.

0.2

0.2

0.3

0.1

0.1

0.01 0.01 0.02 0.02

0.01 0.01

0.01

0.03 0.01 0.02 0.02

0.01

0.01

0.01

0.04 0.01

0.04 0.06

0.01

0.07 0.01 0.01

0.0

0.1

0.1

0.2

0.3

0.3

0.2

0.3

0.2

0.3

0.

0.2

0.2

0.31 0.39 0.37 0.27 0.15 0.29

0.3

0.28 0.24 0.34 0.30 0.27 0.19 0.35 0.41 0.29

0.2

0.3

0.3

0.2

0.1

0.2

0.2

0.3

0.

0.2

0.1

0.3

0.4

0.2

4001 4001 4001

426 426 426

40.35 38.91 38.18

4001 4001 4001 4001 4001 4001

426 426 450 450 450 450

39.50 39.82 38.91 40.95 40.09 40.53

4001 4001 4001 4001 4001 4001 4001 4001 4001

450 462 462 462 462 462 462 476 476

39.84 40.54 38.93 40.38 39.75 39.16 40.14 40.81 39.68

0.04 0.01

0.01 0.012 0.03 0.01 0.03 0.05 0.01 0.04 0.02

0.029

0.011 0.018

Notes: Blank cells are below the detection limit (0.01 wt.%)

33

11.84 11.77 11.24

0.00 0.18 0.04

45.73 48.38 47.50

0.05 0.02 0.05

11.71 10.37 11.58 11.73 11.82 12.20

0.16 0.26 0.26 0.21 0.28 0.26

48.03 47.32 47.87 47.06 47.19 46.98

0.03 0.03 0.05 0.03 0.05 0.07

11.12 12.51 11.09 12.08 11.94 12.28 12.16 12.37 11.76

0.03 0.21 0.12 0.21 0.29 0.00 0.12 0.08 0.29

46.22 46.53 46.29 46.66 46.74 46.96 47.14 47.06 46.93

0.08 0.05 0.05 0.03 0.04 0.04 0.00 0.06 0.03

0.02 0.03 0.01 0.01

0.01

0.02 0.00

0.01

0.08 0.04 0.05 0.02

0.03

0.02 0.02

0.01 0.01 0.01

0.34 0.28 0.24

0.3

0.27 0.34 0.26 0.31 0.27 0.27

0.2

0.33 0.19 0.29 0.26 0.27 0.31 0.19 0.14 0.15

0.3

0.2

0.2

0.3

0.2

0.3

0.2

0.2

0.1

0.2

0.2

0.2

0.3

0.1

0.1

0.1

Sample

Rock type

Table 3. The Sr-Nd isotopic data from the number I Shitoukengde com Rb Sr Sm Nd 87 14 Rb/86Sr 87Sr/86Sr 2σ (ppm) (ppm) (ppm) (ppm)

Drill hole

Depth (m)

Age (Ma)

4001

25

333.9

11.8

56.4

0.60540

0.711000

0.00001

0.35

1.41

4001 4001

38 200

333.9 333.9

3.74 5.39

49.7 79.1

0.21775 0.19717

0.711324 0.710923

0.000009 0.00001

0.32 2.43

1.04 6.6

20.7

1.02604

0.716178

0.000008

0.61

2.16

ST-2 ST-5

Ol websterite Ol websterite Pyroxenite

ST-9

Peridotite

4001

284

333.9

7.34

ST-12

Peridotite

4001

349

333.9

13.3

9.1

4.22909

0.725915

0.000013

0.37

1.51

ST-15

Peridotite

4001

476

333.9

6.63

8.3

2.32258

0.719405

0.000015

0.28

1.15

ST-22

Gabbro

7201

132

424.7

0.712343

0.000006

Gabbro

423

446.0 467.1

0.09083

1501-2

14.00 42.34

0.26227

0.709552

0.000013

0.83 1.76

2.76 6.95

423

21.13

318.6

0.19188

0.709351

0.000016

2.18

7.19

ST-1

S-5

Gabbro

34

Highlights

The Shitoukengde ultramafic rocks with Cu-Ni mineralization formed at 333.9±4.2 Ma, while the Shitoukengde gabbro formed at 424.7±3.7 Ma. The formation of the Shitoukengde ultramafic rocks most likely reflects the opening of the Paleotethys Ocean in the East Kunlun area. The most prospective ultramafic segments of the Shitoukengde are adjacent to gneissic country rocks.

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Graphical Abstract

The Shitoukengde ultramafic rocks formed at 333.9±4.2 Ma (early Carboniferous), while the Shitoukengde gabbro formed at 424.7±3.7 Ma. The formation of the Shitoukengde ultramafic rocks

most likely reflects the opening of the Paleotethys Ocean in the East Kunlun area. In drillhole 4001, sulfide only occurred in the central part of the peridotite, where the earliest fractional crystallization occurred and the whole-rock Ni contents of the peridotite (dunite and lherzolite) and olivine websterite are negatively correlated with the FeO content and positively correlated with the MgO and Fo contents of the olivine. Thus, fractional crystallization fractional crystallization did not play a role in the sulfide saturation process. Otherwise, the Ni content would have increased, and sulfide would have formed during a later period of fractional crystallization. The rocks with higher Ni contents in the peridotite and olivine websterite have lower (87Sr/86Sr)i values, indicating that the rocks with higher Ni contents experienced lower degrees of contamination. The assimilation of marble without sulfides decreases the sulfide solubility of the magmas in the Shitoukengde mafic-ultramafic complex.

36